This present application relates generally to methods, systems, and apparatus for improving the efficiency, performance and operation of combustion turbine engines, which, as used herein and unless specifically stated otherwise, is meant to include all types of combustion turbine or rotary engines, including aircraft engines, the engines of power generating plants and others. More specifically, but not by way of limitation, the present application relates to methods, systems, and apparatus pertaining to fuel delivery in combustion turbine engines.
In general, gas turbine engines include a compressor, a combustor, and a turbine. The compressor and turbine generally include rows of blades that are axially stacked in stages. Each stage includes a row of circumferentially-spaced stator blades, which are fixed, and a row of rotor blades, which rotate about a central axis or shaft. In operation, generally, the compressor rotor blades rotate about the shaft, and, acting in concert with the stator blades, compress a flow of air. The supply of compressed air then is used in the combustor to combust a supply of fuel. The resulting flow of hot combusted gases is expanded through the turbine section. The flow of working fluid through the turbine induces the rotor blades to rotate. The rotor blades are connected to a central shaft such that the rotation of the rotor blades rotates the shaft. In this manner, the energy contained in the fuel is converted into the mechanical energy of the rotating shaft, which, for example, may be used to rotate the rotor blades of the compressor, such that the supply of compressed air needed for combustion is produced, and the coils of a generator, such that electric power is generated.
It is known in the art to preheat a supply of fuel so that it may be delivered at an elevated temperature to the combustor. For several reasons, heated fuel promotes efficient engine operation. At times, it is necessary to vary the temperature of the fuel based on changing fuel characteristics. However, conventional fuel delivery systems have certain limitations that result in difficulties or delays in accurately controlling the temperature of fuel as it is delivered to the combustor. This may lead to the fuel being delivered at temperatures that are not acceptable.
More specifically, combustion turbine engines generally are designed to operate using fuels having certain characteristics relating to heating value. The heating value of a fuel, which also may be referred to as the gross calorific value, gross energy or Wobbe Index rating, generally describes the amount of heat or energy released when the fuel is combusted. In combustion turbine engine applications, though, the amount of energy released by a fuel being combusted through a fuel nozzle at a given pressure ratio may be more accurately described if the temperature at which the fuel is delivered to the nozzle is taken into account. The fuel characteristic that takes into account or compensates for the temperature of the fuel is generally referred to as the Modified Wobbe Index rating or MWI rating. Accordingly, this term will be used in this application; though, its usage is not intended to be limiting. (As used herein, Modified Wobbe Index rating or MWI rating is intended to broadly refer to a fuel measure describing the amount of energy released by a fuel being combusted through a fuel nozzle at a given pressure ratio that takes into account or compensates for the temperature at which the fuel is delivered to the nozzle.) Combustion turbine engines, therefore, are generally designed to operate with fuels that have a specific Modified Wobbe Index rating or fall within a range of acceptable Modified Wobbe Index ratings. This being the case, having the capability to modify or control the temperature of the fuel being delivered to the combustor (thereby modifying or controlling the Modified Wobbe Index rating of the fuel to a preferred range) is a useful way to insure the engine is using fuel that promotes efficient operation and reduces the risk of combustor damage.
However, given the limitations of conventional systems, as discussed in more detail below, variable fuel conditions often result in fuel being delivered to the combustor outside of the acceptable or targeted Modified Wobbe Index rating. Put another way, in conventional systems, fuel often is delivered to the combustor outside of a temperature range that provides the acceptable or targeted Modified Wobbe Index rating. This may result in damage to the combustor and inefficient engine performance. Further, it may result in a turbine engine “runback” situation, during which, generally, the operating system of the engine automatically reduces or cuts engine output to avoid engine damage that may occur because of the fuel not meeting engine specifications. Of course, sudden drops in engine output may come at inopportune moments, such as during peak demand, and result in significant issues in their own right.
In general, there are several reasons for these types of performance shortcomings in conventional fuel delivery systems. One of these is a thermal lag that occurs in manipulating fuel temperature. For example, in systems that include a fuel compressor that heats the fuel through compression and, then, cools a component of the heated flow through the use of a fuel after-cooler, this lag is due at least part by the location of the point at which the two flows are mixed. That is, per conventional systems, the two flows (i.e., the flow that is cooled by the after-cooler and the flow that is not cooled) are mixed in desired amounts so that a fuel supply having a desired temperature is delivered to the combustor. As discussed in more detail below, the location of the mixing point has an impact on the rate at which the temperature of the fuel flow as it enters the combustor may be altered.
Another issue is associated with the timely detection of variable fuel characteristics, such as heating value characteristic, within the fuel supply. The delay in detecting changing conditions makes it difficult to timely determine the appropriate temperature (or temperature range) at which the fuel should be delivered to the combustor. As a result, there is a need for improved methods, systems, and apparatus relating to the delivery of fuel in combustion turbine engines and, particularly, for effectively and efficiently controlling the temperature of the fuel in systems that use a fuel compressor at least intermittently, so that the fuel may be consistently delivered to the combustor at a temperature that is appropriate given its heating value and targeted Modified Wobbe Index rating for the engine.